In many applications, for example in electron microscopes, a strong, although temporally extremely constant, magnetic field is applied to a bounded volume of interest (VOI). Since correspondingly strong magnetic field sources take up a large amount of space, which is usually unavailable at the location of the volume of interest (VOI), a magnetic flux is coupled into a magnetically permeable yoke by means of the magnetic field source and is guided through this yoke into the volume of interest (VOI). This volume of interest is usually implemented as a gap between regions of the yoke that are designed as pole shoes. The area in which the yoke is not designed as a pole shoe usually has the form of a torus.
Different interferences set limits on the temporal constancy of the magnetic field in the volume of interest (VOI) that can be achieved in this case. If a current-carrying coil is used as the magnetic field source, for example, the temporal constancy is limited from the outset by the constancy of the current source used for the current I. In this case, a relative accuracy ΔI/I only in the range 10−7 to 10−8 is achievable with a justifiable amount of effort. The yoke also functions, disadvantageously, as an antenna, which attracts the field lines of interference fields that are electromagnetic or are generated by moving magnetic objects and guides the field lines, together with the intended field, into the volume of interest (VOI). In addition, statistical fluctuations (Barkhausen noise) are impressed upon the magnetic flux transmitted through the yoke as a result of flux jumps in the permeable (magnetically soft) material. Since the permeability of the yoke is temperature-dependent, temperature fluctuations are likewise disadvantageously converted into fluctuations of the magnetic field in the volume of interest (VOI).
A superconducting ring topology, which is disposed within the volume of interest (VOI), for stabilizing the magnetic field is known from U.S. Pat. No. 3,234,435. Disadvantageously, the magnetic field distribution in the volume of interest (VOI) is greatly changed by the ring topology.
In addition, flux tubes move in the ring topology, since the pinning of the flux tubes in strong magnetic fields has only limited effectiveness. This results in further fluctuations of the magnetic field in the VOI.
The object of the invention is, therefore, to provide a device with which the magnetic field in the volume of interest (VOI) can be stabilized without influencing the field distribution.
These objects are achieved according to the invention by a device and by a method for the operation thereof.